The delivery of radio frequency (RF) energy to target regions within tissue is known for a variety of purposes of particular interest to the present invention(s). In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma. RF ablation of tumors is currently performed using one of two core technologies.
The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from the exposed, non-insulated portion of the electrode. This energy translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. PCT application WO 96/29946 and U.S. Pat. No. 6,379,353 disclose such probes. In U.S. Pat. No. 6,379,353, a probe system comprises a cannula having a needle electrode array reciprocatably mounted therein. The individual electrodes within the array have spring memory, so that they assume a radially outward, arcuate configuration as they are advanced distally from the cannula. In general, a multiple electrode array creates a larger lesion than that created by a single needle electrode.
In theory, RF ablation can be used to sculpt precisely the volume of necrosis to match the extent of the tumor. By varying the power output and the type of electrical waveform, it is possible to control the extent of heating, and thus, the resulting ablation. However, the size of tissue coagulation created from a single electrode, and to a lesser extent a multiple electrode array, has been limited by heat dispersion. As a consequence, when ablating lesions that are larger than the capability of the above-mentioned devices, the common practice is to stack ablations (i.e., perform multiple ablations) within a given area. This requires multiple electrode placements and ablations facilitated by the use of ultrasound imaging to visualize the electrode in relation to the target tissue. Because of the echogenic cloud created by the ablated tissue, however, this process often becomes difficult to accurately perform. This process considerably increases treatment duration and patent discomfort and requires significant skill for meticulous precision of probe placement.
In response to this, the marketplace has attempted to create larger lesions with a single probe insertion. Increasing generator output, however, has been generally unsuccessful for increasing lesion diameter, because an increased wattage is associated with a local increase of temperature to more than 100° C., which induces tissue vaporization and charring. This then increases local tissue impedance, limiting RF deposition, and therefore heat diffusion and associated coagulation necrosis. In addition, patient tolerance appears to be at the maximum using currently available 200 W generators.
To a large extent, the size and nature of an ablation lesion depends on how the electrode element(s) are arranged. In one arrangement, RF current may be delivered to an electrode element (whether a single electrode or electrode array) in a monopolar fashion, which means that current will pass from the electrode element to a dispersive electrode attached externally to the patient, e.g., using a contact pad placed on the patient's flank. In another arrangement, the RF current is delivered to two electrode elements in a bipolar fashion, which means that current will pass between “positive” and “negative” electrode elements. Bipolar arrangements, which require the RF energy to traverse through a relatively small amount of tissue between the tightly spaced electrodes, are more efficient than monopolar arrangements, which require the RF energy to traverse through the thickness of the patient's body. As a result, bipolar electrode arrays generally create larger and/or more efficient lesions than monopolar electrode arrays. To provide even larger lesions, it is known to operate two electrode arrays in a bipolar arrangement.
Thus, to a certain extent, the use of bipolar electrode arrangements has eliminated the need to “stack” ablations when treating a tumor. The ability to provide uniform heating and the creation of homogenous tissue lesions, however, is particularly difficult with bipolar devices. For example, the two bipolar electrodes may be placed in regions with quite different perfusion characteristics, and the heating around each pole can be quite different. That is, one pole may be located adjacent to a large blood vessel, while the other pole may be located adjacent to tissue, which is less perfused. Thus, the pole located in the less perfused tissue will heat the tissue immediately surrounding the electrode much more rapidly than the tissue surrounding the opposite polar electrode is heated. In such circumstances, the tissue surrounding one pole may be preferentially heated and necrosed, while the tissue surrounding the other pole will neither be heated nor necrosed sufficiently.
In the case where two electrode arrays are used, if the distance between the electrode arrays is too great in an attempt to ablate a longer tissue volume, the energy transmitted between the electrode arrays may thin and not fully ablate the intermediate tissue. As a result, an hour-glass shaped ablation, rather than the desired uniform circular/elliptical ablation, may be created. Also, because the electrode arrays are, in effect, three-dimensional, portions between the electrode arrays will be closer together than other portions of the electrode arrays, thereby causing a non-uniform current density between the electrode arrays, resulting in a non-uniform ablation. Besides lacking the ability to produce predictable homogenous lesions, bipolar arrangements, which are designed to ablate tissue between the electrodes, are not well-suited for simultaneously ablating multiple tissue regions.
In situations where it is desired to produce large homogenous lesions or simultaneously ablate multiple tissue regions, it is known to arrange multiple probes in a monopolar fashion (i.e., the RF energy generated by each probe is conveyed to a dispersive electrode attached to the skin of the patient. In this case, current flows from each probe to the ground pad. A drawback to this approach is that simultaneously supplying power to multiple probes taxes the power output by the RF generator, which may cause insufficient heating around the probes. Also, because the tissue adjacent the probes is non-uniform (e.g., one probe may be adjacent a blood vessel), the heating pattern created by the probes will be non-uniform, thereby making it difficult to predict the nature of the resulting lesion.
To address these drawbacks, it is known to use an ablation system that sequentially switches ablation energy between probes, so that at any given time, ablation energy is supplied to only one probe. While this switching technique may result in a more efficient and predictable lesion, it is believed that, during any given time period, the tissue adjacent the probes to which the ablation energy is not currently supplied temporarily cools-especially when the switching speed between the probes is relatively slow, e.g., a few seconds. As a result, the cooled tissue must be reheated when power is again supplied to the adjacent probes, thereby losing some efficiency in the ablation process.
For this reason, it would be desirable to provide improved multi-probe electrosurgical methods and systems for more efficiently ablating tumors in the liver and other body organs.